Well, if all the herbivores disappeared, multiple things could occur. Without anyone keeping plants in check, plants may grow too abundant and may be harmful to the food web. Not only that, but predators that eat herbivors might starve without food. On the plus side, natural resources like water would increase without herbivores drinking/using it.
Carnivores wouldn’t have their food supply anymore and could starve. Plants could also grow out of control without herbivores to eat them and keep ecosystem in balance
